The ratio of duration of systole to overall cardiac cycle duration (ED%) reflects the specifics of the cardiac cycle.

Systole is the time of tension and intense activity of the heart muscle; diastole is the time of relaxation, recovery, and blood supply. The approximate normal systole-pulse wave length ratio is 1/3 or 33%. As the heart rate grows, the cardiac cycle shortens mostly due to diastole.

The duration of systole sometimes rises to 50% and over, entering the zone of myocardial instability.

To estimate this index, methods have been developed to convert the duration of systole into the value expected at a pulse rate of 60 beats per minute.

The ED% values are characterized by the following qualitative intervals:

• <25%: Express decrease in the duration of systole. Increased risk of exacerbations
• 25-29%: Moderate decrease in the duration of systole. Trend toward higher risk of exacerbations
• 29-40%: Normal duration of systole
• 40-43%: Moderate increase of the duration of systole against normal values. Heart rate stability deterioration, risk of arrhythmia
• >43%: express increase of the duration of systole against normal values Significant heart rate stability deterioration, higher risk of arrhythmia

The extreme values of this indicators can be shifted by +1% for women and by -1% for men. Alternatively, one can be guided by the averages.

An increased duration of systole is accompanied by lower heart rate stability and higher risk of arrhythmia. A significant decrease in the duration of systole may entail a higher risk of cardiovascular complications.

Here are the possible reasons behind the increased duration of systole:

• Myocardial infarction in the past
• Impaired myocardial contractility
• Enlargement of the left ventricle
• Congenital traits
• Administration of certain drugs
• Alcohol abuse
• Lack of potassium, magnesium
• Low-protein diet